An AR-pattern firearm (which includes, but is not limited to, the AR15, AR10, M16, M4, and clones of these) has separable upper and lower receivers. The lower receiver houses the fire control group of parts, including the hammer, trigger, disconnector, and, when capable of fully automatic fire (i.e., a machine gun), an auto sear. The upper receiver houses a longitudinally reciprocating bolt and bolt carrier. In a typical configuration, the bolt carrier extends substantially the entire length of the upper receiver and moves rearwardly beyond the confines of the upper receiver when the action cycles. In this typical configuration, a recoil spring and buffer are housed in an extension positioned reward of the upper receiver.
In compact versions, in which the recoil spring is housed within the upper receiver, usually actuated by a gas piston system, or having a simple blowback bolt, such in a pistol caliber carbine, there is no need for the bolt carrier to extend the full length of the upper receiver or to reciprocate beyond the confines of the upper receiver. Operating systems of this type allow for the use of a folding stock and/or eliminate the need for an extension to house the recoil spring and buffer in a pistol configuration. However, these compact bolt carriers are unable to trip the auto sear, which has to be tripped just prior to the bolt carrier and bolt returning to its full in-battery position, where it is situated in a standard lower receiver without significant modification to the shortened bolt carrier.
Others have proposed solutions to this challenge with “lost motion” or “slip” linkage devices that attach to and require modification of the upper receiver. For example, U.S. Pat. No. 8,667,882, issued Mar. 11, 2014, shows one such device which attaches to the upper receiver and reciprocates on elongated grooves or channels machined on the interior of the upper receiver. Another example is shown by U.S. Pat. No. 9,151,558, issued Oct. 6, 2015, which uses a sear trip bar that is held slidably captive in a groove machined on one side of the interior of the upper receiver. In rim-fire conversion devices for AR-platform firearms, auto sear trip bars have been integrated into or attached to an elongated structure that replaces the standard bolt carrier, or that are carried within the upper receiver. One example is shown in U.S. Pat. No. 3,776,095, issued Dec. 4, 1973.